With respect to baked products of melamine and phosphoric acid, melamine orthophosphate, melamine pyrophosphate and melamine polyphosphate have, for example, been known, and there have been many reports on processes for their production. JP-A-61-126091 discloses a process for producing melamine condensed phosphate which comprises reacting condensed phosphoric acid and melamine in a solid phase substantially in the absence of an aqueous medium under a temperature condition of from naturally generated heat temperature to 170.degree. C.
It is known to employ urea as a condensation agent in the production of a polyphosphate. For example, JP-B-53-2170 discloses a process for producing a polyphosphoric acid amide (amide polyphosphate) containing amide-type nitrogen, which is obtainable by using ammonium orthophosphate, orthophosphoric acid, condensed phosphoric acid, phosphoric anhydride, urea phosphate, or a mixture thereof, as a phosphoric acid source, and a cyanamide compound such as melamine, dicyan cyanamide, guanidine or guanyl urea, as a nitrogen source, and subjecting a mixture thereof to a heat condensation reaction in the presence of urea, urea phosphate or a mixture thereof, as a condensation agent. As the production conditions, it is disclosed to carry out the heat condensation in a ratio of urea/phosphoric acid (as H.sub.3 PO.sub.4)/cyanamide compound=0.8-1.5/1/0.05-1 (molar ratio) in an ammonia gas atmosphere at a temperature of from 150 to 350.degree. C. for from 10 minutes to 5 hours, preferably from 1 to 4 hours.
A.C.S. Symposium Series No. 425 "Fire and Polymers", chapter 15, p. 211-238, American Chemical Society, Washington, D.C., 1990, discloses that melam ultraphosphate is formed by heating melamine phosphate [melamine/phosphorus atom=1/1 (molar ratio)] at a temperature of from 330 to 410.degree. C.
JP-A-8-231517 discloses a process for producing a melamine sulfate composition characterized by reacting sulfuric acid and melamine in a molar ratio of 1:0.1 to 1, as an aminotriazine sulfate composition having water resistance.
With respect to a baked product of melamine and sulfuric acid, the above-mentioned A.C.S. Symposium Series No. 425 "Fire and Polymers", chapter 15, p. 211-238, American Chemical Society, Washington, D.C., 1990, discloses melamine sulfate [melamine/sulfur atom=1/1 (molar ratio)] and that dimelam pyrosulfate will be formed by heating it at a temperature of from 300 to 400.
There has been no report on a baked product comprising three components of melamine, phosphoric acid and sulfuric acid.
Many proposals have been made up to present with respect to utilization of melamine phosphate as a flame retardant. For example, JP-A-53-49054 discloses a polyamide resin composition having the flame retardancy improved by adding an inorganic filler and melamine phosphate to a polyamide.
JP-A-61-126091 discloses that melamine condensed phosphate is effective as a flame retardant for a thermoplastic resin such as a polyester, a polyamide or a polyolefin, a thermosetting resin such as phenol or urethane epoxy, or a cellulose material.
With respect to utilization of melamine sulfate as a flame retardant, JP-A-54-47750 discloses that melamine sulfate [melamine/sulfur atom=2/1 (molar ratio)] is used as a flame retardant for polyamide.
The melamine polyphosphate or the melamine condensed phosphate as disclosed in the above-mentioned prior art can hardly be used effectively as a flame retardant for a resin having a high molding temperature, since desorption of melamine is substantial when heated at a temperature of at least 300.degree. C. Further, the above-mentioned melamine sulfate can hardly be used effectively as a flame retardant for a resin having a high molding temperature, since it produces ammonium hydrogensulfate when heated at a temperature of at least 300.degree. C.
The present invention is to overcome the drawbacks of the above-mentioned prior art and to present a 1,3,5-triazine derivative salt of a polyacid comprising phosphorus, sulfur and oxygen, which is useful for a wide range of applications as a flame retardant and which is produced from melamine, phosphoric acid and sulfuric acid as starting materials, and a process for its production.